Nuclear Fusion, 38(5), 673-680 (1998).
Smith, P. C., Ruzic, D. N.
The phenomenon of ion induced sputtering is integral to many applications. In magnetically confined fusion, this sputtering is important for both the lifetime of the plasma facing components and the contamination of the plasma. A method has been developed to obtain both the angular distribution and the total sputtering yield. The total yield is determined by collecting the sputtered material on a quartz crystal microbalance. The sputtered material is also collected on a pyrolytic graphite collector plate. By mapping the concentrations of the sputtered material on this plate, both the polar and the azimuthal angular distribution can be determined. Utilizing this set-up, data have been obtained for 10 to 700 eV D+ on beryllium at a 45° angle of incidence to the normal. Subthreshold sputtering (0.004 ± 0.003 at 10 eV) has been observed. These data are some of the first to become available, especially at the lower energies.
J. Vac. Sci. Technol., A, 16(2), 624-627 (1998).
Hayden, D. B., Juliano, D. R., Green, K. M., Ruzic, D. N., Weiss, C. A., Ashtiani, K. A., Licata, T. J.
A rotating magnet dc planar magnetron with a 33-cm diameter aluminum target is coupled with a secondary plasma source to ionize the sputtered metal neutral flux to control the angular distribution of the flux arriving at the surface of the substrate. For this purpose, a radio-frequency (rf) plasma is created between the sputtering target and substrate by a three-turn coil located in the vacuum chamber. The rf plasma increases the electron temperature and density, which results in significant ionization of the neutral metal flux from the sputtering target. By applying a small negative bias to the substrate, metal ions are drawn to the substrate at normal incidence. A gridded energy analyzer and a quartz crystal microbalance (QCM) were used to determine the ion and neutral deposition rates. From this, the ionization fraction of the flux incident onto the QCM is determined.
Review of Scientific Instruments, 68, 4555-4560 (1997).
Green, K. M., Hayden, D. B., Juliano, D. R., Ruzic, D. N.
A diagnostic which combines a quartz crystal microbalance (QCM) and a gridded energy analyzer has been developed to measure the metal flux ionization fraction in a modified commercial dc magnetron sputtering device. The sensor is mounted on a linear motion feedthrough and embedded in a slot in the substrate plane to allow for measuring the uniformity in deposition and ionization throughout the plane of the wafer. Radio-frequency (rf) power is introduced through a coil to ionize the Al atoms. The metal flux ionization fraction at the QCM is determined by comparing the total deposition rate with and without a bias that screens out the ions, but that leaves the plasma undisturbed. By varying the voltage applied to the grids, the plasma potential is determined. At a pressure of 35 mTorr, a magnetron power of 2 kW, and a net rf power of 310 ± 5 W, 78 ± 5% ionization was found.
Fusion Engineering and Design, 37, 332-337 (1997).
Brooks, J. N., Ruzic, D. N., Hayden, D. B.
Sputtering erosion is a key performance issue for beryllium coatings in future fusion reactors. Past analysis of this subject for various edge plasma regimes—as briefly reviewed here—shows acceptable erosion only for low duty factor devices. Erosion analysis is now focusing on a very low boundary temperature (≤2 eV) ‘detached plasma’ regime for the International Thermonuclear Experimental Reactor (ITER) Vertical Director design. Erosion and tritium codeposition calculations are made for this regime using a coupled WBC/REDEP/DEGAS+ code analysis. The analysis includes calculations of low energy, oblique incidence, sputtering yields using the VFTRIM-3D code calibrated to fit recent data on D–Be sputtering. We find that beryllium performs better than carbon for detached conditions due to the absence of chemical sputtering. Beryllium performs worse than vanadium or tungsten. Net erosion rates of beryllium coated first wall components, due mainly to physical sputtering by charge exchange D–T neutrals, vary from 
0 to 2 cm per burn-year along the wall. Tritium co-deposition in redeposited beryllium surface layers may be high.
J. Nuclear Materials, 241-243, 294-298 (1997).
Brooks, J. N., Causey, R., Federici, G., Ruzic, D. N.
We analyzed sputtering erosion and tritium codeposition for the ITER vertical target divertor design using erosion and plasma codes (WBC/REDEP/DEGAS+) coupled to available materials data. Computations were made for a beryllium, carbon, and tungsten coated divertor plate, and for three edge plasma regimes. New data on tritium codeposition in beryllium was obtained with the tritium plasma experiment (TPE) facility. This shows codeposited H/Be ratios of the order of 10% for surface temperatures ≤ 300°C, beryllium thereby being similar to carbon in this respect. Hydrocarbon transport calculations show significant loss (10–20%) of chemically sputtered carbon for detached conditions (Te ≈ 1 eV at the divertor), compared to essentially no loss (100% redeposition) for higher temperature plasmas. Calculations also show a high, non-thermal, D-T molecular flux for detached conditions. Tritium codeposition rates for carbon are very high for detached conditions ( 20 g T/1000 s discharge), due to buildup of chemically sputtered carbon on relatively cold surfaces of the divertor cassette. Codeposition is lower ( 10X) for higher edge temperatures ( 8–30 eV) and is primarily due to divertor plate buildup of physically sputtered carbon. Peak net erosion rates for carbon are of the order of 30 cm/burn yr. Erosion and codeposition rates for beryllium are much lower than for carbon at detached conditions, but are similar to carbon for the higher temperatures. Both erosion and tritium codeposition are essentially nil for tungsten for the regimes studied.
J. Nuclear Materials, 241-243, 1170-1174 (1997).
Ruzic, D. N., Smith, P. C., Turkot Jr., R. B.
The angular-resolved sputtering yield of Be by D+ was predicted and then measured. An ion beam at 100, 300, 500 and 700 eV from a Colutron ion source was focused onto S-65 C grade Be samples. The sample was exposed in situ to a 350 V dc D plasma to remove oxide, load the surface with D and more-nearly simulate the surface which would be found during steady-state fusion device operating conditions. The angular distribution of the sputtered atoms was measured by collection on a highly ordered pyrolytic graphite witness plate. The areal density of Be (and BeO, after exposure to air) was then measured using a scanning Auger spectrometer. Total deposition was measured by deposition onto a quartz crystal oscillator placed alongside the witness plate. A three-dimensional version of vectorized fractal TRIM (VFTRIM3D), a Monte-Carlo computer code which includes surface roughness characterized by fractal geometry, was used to predict the angular distribution of the sputtered particles and a global sputtering coefficient. One-quarter million trajectories were simulated to determine the azimuthal and polar angle distributions of the sputtered atoms. A fractal dimension of 2.05, and a surface binding energy of 3.38 eV, both standard values for Be, were used. Results show reasonable agreement between the code and experimental values for total yield with the experimental yields somewhat lower. The measured angular distribution is broader (less forward peaked) than predicted by the computer simulation.
Phys. of Plasmas, 3(11), 4084-4094 (1996).
Stotler, D. P., Skinner, C. H., Budny, R. U., Ramsey, A. T., Ruzic, D. N., Turkot Jr., R. B.
Monte Carlo neutral transport simulations of hydrogen velocities in the Tokamak Fusion Test Reactor (TFTR) [K. M. McGuire et al., Phys. Plasmas 2, 2176 (1995)] are compared with experiment using the Doppler-broadened Balmer-
spectral line profile. Good agreement is obtained under a range of conditions, validating the treatment of charge exchange, molecular dissociation, surface reflection, and sputtering in the neutral gas code DEGAS [D. Heifetz et al., J. Comput. Phys. 46, 309 (1982)]. A residual deficiency of 10–100 eV neutrals in most of the simulations indicates that further study of the energetics of H
dissociation for electron energies in excess of 100 eV is needed.
J. Vac. Sci. Technol., 14(4), 2094-2101 (1996).
Kushner, M. J., Collison W. Z., Ruzic, D. N.
During plasma etching and deposition of semiconductor materials, it is desirable to have separate control over the magnitude and energy of the ion flux onto the substrate. This control is difficult to achieve in reactive ion etching discharges since the radio frequency (rf) voltage applied to the substrate both generates the ions and accelerates the ions into the substrate. High plasma density devices such as electron cyclotron resonance and inductively coupled plasma reactors achieve this control by having separate power sources for ionization and ion acceleration. In this article, we present results from a computational study of an electron beam controlled rf discharge in which the production and acceleration of ions are similarly separately controlled. Ionization is dominantly produced by injection of an electron beam into the reactor. Ion acceleration is determined by a separate rf bias applied to the substrate. The limits of e-beam voltage, current, and rf bias voltage for which this separate control can be achieved will be discussed.
J. Vac. Sci. Technol., B, 14 (3), 1819-1827 (1996).
Rossnagel, S. M., Nichols, C., Hamaguchi, S., Ruzic, D. N., Turkot, R.
Thin, nearly conformal films are required for semiconductor applications to function as diffusion barriers, adhesion layers and seed layers within trenches and vias. The deposition of high mass refractory films with conventional, noncollimated magnetron sputtering at low pressures shows better-than-expected conformality which is dependent on the degree of directionality of the depositing atoms: the conformality increases as the directionality increases. The primary cause appears to be a strongly angle-dependent reflection coefficient for the depositing metal atoms. As the deposition is made more directional by increasing the cathode-to-sample distance, the depositing atoms are more likely to reflect from the steep sidewalls, leading to better conformality as well as a less columnar film structure.
Plasma Science, 24, 81-82 (1996).
Ruzic, D. N.
Three frames from a molecular dynamics video which depicts the interaction of a 10-eV hydrogen atom with a rough nickel surface at 300 K are shown. The potential energy and velocity vectors of all the atoms are shown through imaginative graphics. A reflection, an absorption, and a re-entrant flight are shown.
